1) Field of the Invention
The present invention relates to a polyhydroxycarboxylic acid which is a useful biodegradable polymer as a substitute for medical materials or general purpose resins and a process for preparing the polyhydroxycarboxylic acid directly from hydroxycarboxylic acid by dehydration condensation. Hydroxycarboxylic acid, particularly lactic acid, is widely distributed in the natural world and is harmless to animals, plants, men and beasts. Polylactic acid which is a polymer of the lactic acid can be hydrolyzed with relative ease in the presence of water and is also hydrolyzed and absorbed in vivo. Thus polylactic acid has attracted attention as a polymer which can be applied to the above uses.
2) Description of the Related Art
Polylactic acid or polyglycolic acid which is a polyhydroxycarboxylic acid has been generally prepared by ring opening polymerization of lactide or glycolide which is a cyclic dimer of lactic acid or glycolic acid.
U.S. Pat. No. 2,703,316 has disclosed a preparation process of poly-D,L-lactic acid. D,L-lactic acid is initially converted to an oligomer, lactide is isolated at 200.degree..about.250.degree. C. under reduced pressure and successively recrystallized a few times from ethyl acetate. Racemi-lactide thus obtained has a melting point of 120.degree. C. or above and can be converted to poly-D,L-lactic acid by ring-opening polymerization. Poly-D,L-lactic acid obtained has an inherent viscosity (.eta.) of 0.45 dl/g or more and can be processed into a tough film or monofilament. It has also been described that the polymer obtained by direct condensation of lactic acid is brittle and cannot be stretched.
U.S. Pat. No. 2,758,987 has described a preparation process of poly-L-lactic acid having an inherent viscosity (.eta.) of 0.4 dl/g or more from L,L-lactide which is obtained in the same manner from L-lactic acid and has a melting point of 94.degree. C. or above.
However, preparation of lactide or glycolide which is suited for a material of polymer requires much labor and expenses such as distillation and recrystallization and is unfavorable from an economic standpoint. The process cannot be carried out in the case of copolymerizing hydroxycarboxylic acid which does not form acyclic lactone such as lactide or glycolide.
On the other hand, a direct polycondensation of hydroxycarboxylic acid such as lactic acid and glycolic acid is a successive reaction similarly to an esterification reaction of dibasic acid and polyhydric alcohol. Molecular weight of the product increases with reaction time. Since generated water acts to reduce the molecular weight of the polycondensate through hydrolysis it has been required to remove generated water out of the reaction system in order to obtain high molecular weight polyhydroxycarboxylic acid such as polylactic acid or polyglycolic acid.
Japanese Laid-Open Patent SHO 59-96,123 has disclosed a process for preparing polylactic acid having a molecular weight of 4,000 or more by carrying out a condensation reaction at temperature of 220.degree..about.260.degree. C. under reduced pressure of 10 mmHg or less in the absence of a catalyst.
U.S. Pat. No. 4,273,920 has disclosed a copolymer of lactic acid and glycolic acid obtained by conducting dehydration condensation in the presence of an ion exchange resin catalyst and successively removing the catalyst. The copolymer contains substantially no catalyst and has an inherent viscosity (.eta.) of 0.08.about.0.30 dl/g and an average molecular weight of 6,000.about.35,000.
However, a high temperature of 180.degree. C. or above is required in order to obtain high molecular weight polymer by the above processes. The polymer obtained under such condition has problems such as discoloration and the presence of impurities formed by decomposition of the polymer. These problems have not yet been solved with polymers having an average molecular weight of 15,000 or more.
The molecular weight range of the polymer obtained by these processes is also restricted and thus a polymer which can exhibit sufficient strength in the form of a film or monofilament cannot be obtained.